1. Field of the Invention
This invention relates to a fiber-optic sensor array for use in vertical seismic profiling.
2. Description of the Prior Art
Vertical seismic profiling (VSP) is a method of determining acoustic wave characteristics of rock layers in situ. The method includes lowering one or several sensors in a well bore to a preselected depth. Use of several spaced-apart sensors fastened to the single cable allows coverage over a preselected depth interval. A seismic signal generated at or near the surface of the earth propagates through the earth and is received by the sensors. The sensors convert the acoustic energy to analog electrical impulses which are transmitted by the cable to the surface to a suitable processor and recorder.
Conventional VSP sensors are of many types including piezoelectric hydrophones, moving-coil geophones, variable-reluctance geophones, and magnetostrictive transducers. When excited by acoustic energy the sensors produce electrical analog signals that are representative of the time-variant pressure changes or the particle-velocity in the borehole fluid or sidewalls. Several obstacles arise when using the aforementioned sensors in deep-well velocity surveys.
Conventional pressure-sensitive sensors, such as bender-type piezoelectric hydrophones may become inoperative when subjected to high pressures. The piezoelectric wafers are compressed against a backing plate, preventing the sensor from generating a signal. In extreme cases the active element is crushed.
Velocity-sensitive sensors, such as the GeoSpace WLS-1050 Geophone, are enclosed in watertight sondes so that the sensors are not responsive to hydrostatic pressures. These tools must then, be in direct contact with the well-bore wall to operate efficiently.
A primary disadvantage of using velocity-sensitive sensors in deep-well velocity surveys is one of space. Each sonde is large, often ten feet or more long and several inches in diameter. Usually, each sonde requires its own power supply and may require a separate channel. Because of limitations of the well-bore diameter, and the space required at the surface for the cable drawworks, only a limited number of sondes can be lowered down the well bore. That circumstance limits the sensor density over the desired survey interval.
Optical fibers and waveguides have been used to detect acoustic signals. J. A. Bucaro et al., teaches such a device in U.S. Pat. No. 4,238,856. In another U.S. Pat., No. 4,320,475 to Leclerc et al., a pressure sensor uses a reference-beam fiber and a sensing-beam fiber. A coherent beam of radiation is split to pass through the two fibers. They are then recombined and are allowed to interfere at a detector to measure the amount of acoustic energy exciting the sensing optic fiber as a function of phase shift of the two beams. A disadvantage of the above sensors is that they use signal reference-beam fiber paths which result in relative phase shifts between the two beams that are not related to the signal phase shifts. These spurious shifts are caused by differences in the acoustical/mechanical environment.
Optical fibers have been used in data transmission. U.S. Pat. No. 4,302,835 issued to McMahon teaches a method and apparatus for time-multiplexing signal pulses propagating along a bidirectional optical transmission line from a plurality of successive sensors. By use of evanescent couplers the sensors are coupled in parallel to a transmission line. Pulses transmitted from a source are coupled into the respective sensors where the pulses are data-modulated. The data-modulated pulses are coupled back into the bidirectional line and are returned to a receiver where they are demultiplexed on a time-of-arrival basis.